© EDP Sciences 2016

1 Introduction

In all countries with nuclear installations detailed emergency management strategies have been developed in the past. In nearly all cases such strategies are based on accident scenarios where the duration of the release of radionuclides to the environment is limited to some hours or at maximum a few days. The Fukushima-Daiichi accident has demonstrated the possibility of long-lasting releases of radionuclides from a nuclear power plant (NPP) over several weeks. The overall objective of the work package described in this paper is to prepare long-lasting environmental ST's for typical reactor units that are in operation in European countries with the aim to test the current off-site nuclear emergency planning in European countries with accident scenarios based on lessons learned from the Fukushima accident and to derive recommendations how to improve the current planning. The tests should demonstrate whether protective measures foreseen in the current emergency planning could adequately reduce the radiological consequences of NPP accidents with long lasting severe releases similar to the release from the Fukushima-Daiichi NPP.

2 Source terms

The key aspect of this work is the investigation of hypothetical source terms leading to long-lasting releases of radionuclides and which are applicable to nuclear installations in various European countries. Source terms are either be developed using computer codes for modelling the progression of severe accidents (SA) or be derived from existing studies of potential releases from nuclear installations (including probabilistic safety assessment results). The source terms have been derived for NPPs with Pressurized Water Reactors (PWR or VVER) and Boiling Water Reactors (BWR) that are in operation in Germany, Slovakia, Finland and Spain. These source terms correspond to the International Nuclear and Radiological Event Scale (INES) – categories: 5 (accident with wider consequences), 6 (serious accident) and 7 (major accident). All data regarding the described environmental ST's are taken from the report (Gering et al., 2014).

It should be mentioned that in case of Fukushima-Daiichi NPP accident the Nuclear Industrial and Safety Agency (NISA) in Japan declared the accident at level 7 (UNSCEAR, 2013). The accident was a consequence of earthquake of magnitude 9 along the Japan trench and the following tsunami. Significant release of the radioactive materials to the atmosphere began on 12th of March and the release rate varied considerably in magnitude within the following one week, with marked increases associated with particular events at each unit (e.g. venting of primary containments and hydrogen explosions). After the first week, the rates of release gradually declined. By the beginning of April, the release rates had fallen to a thousandth or less of the release rates that occurred during the first week of the accident. The estimates of the “total” release to atmosphere of ¹³¹I fell within the range of 100–500 PBq (1 × 10¹⁵) and those of ¹³⁷Cs in the range of 6–20 PBq (UNSCEAR, 2013). These ranges comprised about 2–8% of the total inventory of ¹³¹I and about 1–3% of the total inventory of ¹³⁷Cs in the three operating reactors (Units 1–3) at the time of the accident. The total releases of ¹³³Xe to the atmosphere assumed by the Committee are 7 300 PBq (UNSCEAR, 2013). Estimated direct releases to the ocean of ¹³¹I fell within the range of 10–20 PBq and those of ¹³⁷Cs in the range 3–6 PBq (UNSCEAR, 2013; note that these values does not comprise radionuclides initially released to the atmosphere and subsequently deposited onto the ocean surface).

2.1 Potential source terms for German NPP's

The source terms considered in this study for German NPP's have been derived by GRS (Gesellschaft für Anlagen- und Reaktorsicherheit). All three environmental ST's (marked as FKA, FKI, and FKF) are based on accident scenarios leading to complete core damage followed by failure of the RPV leading to Molten Corium Concrete Interaction (MCCI) in the reactor cavity.

These environmental ST's have been derived in the frame of a research project by GRS (GRS-A-3580) to provide up to date source terms for Real-time On-line DecisiOn Support system (RODOS) in 2010. The GRS performed Probabilistic Safety Assessment studies – Level 2 (PSA L2) for German BWR's and the environmental ST's for RODOS were selected “according to criteria like frequency, extent of the source term and speed of the accident sequence” (GRS-A-3580). The main characteristics of selected SA scenarios and associated ST's are presented in Table 1.

2.2 Potential source terms for Slovakian NPP's

Three environmental ST's which are considered in this study for Slovak NPP's have been derived by VUJE (VUJE, Inc.) and all are based on accident scenarios leading to core damage. The ST's have been selected from the PSA L2 study for Unit 3 of the Mochovce NPP (VVER-440/213 unit with vacuum bubble-condenser system) and have been calculated by means of the validated integral SA code MELCOR1.8.5. The considered three SA scenarios with associated environmental ST's have been calculated for ∼2 days of release duration (in accordance with PSA L2 methodology) and comprise scenarios with different initiating events and mainly of different timing for begin of major release of radionuclides to environment due to containment failure.

To simulate long lasting release like in Fukushima-Daiichi accident the original ST's have been extended to duration of ∼8 days in this study, i.e. all time periods of the original ST's as well as timing of important events (e.g. time of CONT failure since the accident initiation) have been stretched in time by a factor of 4 resulting in the new ST's marked as STC6-8d, STC10-8d and STC30-8d.

It should be mentioned that all VVER-440/213 units (4 + 2 under finalisation) are already equipped with Severe Accident Management (SAM) systems devoted to mitigate ST and radiological consequences in case of SA occurring. SAM involves, e.g. system dedicated for primary circuit depressurization, diesel generator for SAM (DG SAM), dedicated JNR system for injection of water to primary circuit and containment spray system from external source of borated water (3 × 500 m³ of water; as minimum 1 000 m³ is available, but later additional water can be added). In the original analyses the usage of JNR system for water delivery into RPV was not supposed. Therefore, the justification of the timing prolongation can be made through assumption that JNR system was used for water delivery into RPV but not on sufficient level, i.e. preventing or ending of core melt was not successful but process of core degradation was significantly prolonged (it is supposed that after 8 days from accident initiation the degraded core – corium is successfully flooded, cooled down and stabilized).

The main characteristics of prolonged three SA scenarios and associated ST's are presented in Table 2.

2.3 Potential source terms for Finnish NPP's

Two environmental ST's which are considered in this study for Finnish NPP's have been provided by STUK's Nuclear Reactor Regulation department's Risk Analysis office and are based on accident scenarios leading to core damage. The ST's have been selected from the PSA L2 study, one for PWR Unit and one for BWR Unit. The main characteristics of these two SA scenarios and associated ST's are presented in Table 3.

2.4 Potential source terms for Spanish NPP's

The environmental ST for Spanish NPP's has been estimated by the Spanish Regulatory Authority (CSN) and has been provided for this study by CIEMAT (Centro de Investigaciones Energéticas, Médioambientales y Tecnológicas). Source term MA-SBO-173 is based on an accident scenario leading to complete core damage and is at present, the only potential long lasting ST considered for Spanish NPP's. Although all the Spanish NPP's keep updated their PSA, including those related to internal events at power (PSA L2), the framework in which the MA-SBO-173 source term was derived is the National Action Plan resulting from the Stress Tests after Fukushima. The main characteristics of the SA scenario and associated ST (MA-SBO-173) are presented in Table 4.

3 Conclusions

In the work package described in this paper the SA environmental ST's have been investigated that mostly leading to long-lasting releases of radionuclides and which are applicable to nuclear installations in various European countries. The main characteristics of these environmental ST's are the following:

• Three reactor types have been considered, these are PWRs, BWRs and VVER-440/213.

• The accident scenarios leading to the accidental releases of radioactivity include core melt followed by total core damage scenarios in all cases, combined with CONT damage in 7 cases, while in the remaining 3 cases releases occur through venting.

• The amount of ¹³¹I released in the 9 scenarios varies between 3 PBq and 300 PBq.

• The duration of the releases varies between 22 h and 188 h (∼8 days).

• Most of the releases would be rated as INES 7, with only 2 cases rated as INES 6 and one case as INES 5.

Results of this work (i.e. selected long lasting environmental ST's) have been used as an input for the knowledge data bases of the review of emergency preparedness for long lasting releases, i.e. for development of the accident scenario used for radiological consequence assessment and evaluation assessment.

Acknowledgement

The research leading to these results has received funding from the European Atomic Energy Community Seventh Framework Programme FP7/2012-2013 under grant agreement 323287.

References

All Tables